Why fSpy addon can't be enabled
New answer (after getting feedback in the comments):
The file you are trying to install is the wrong one (i.e. the repository downloaded as a zip file). Try with the release file instead.

Old answer:
I suspect that it is in fact installed but not showing up in the list because it is not enabled yet (your screenshot is only showing enabled add-ons).

First, uncheck the checkbox that says "Enabled Add-ons Only" to be able to find the add-on, then check the checkbox for the add-on to enable it.